Nitric oxide is unusual among animal signal molecules in that it __________.

• A. is a gas that diffuses directly across cell membranes
• B. binds exclusively to intracellular steroid receptors
• C. acts only through G protein-coupled receptors
• D. is stored in vesicles before being released

Answer: (A)

Verified step by step guidance

1. Understand the role of nitric oxide (NO) as a signaling molecule in animals. Nitric oxide is unique because it is a gas, unlike most signaling molecules that are proteins or other complex molecules.
2. Recall that gases like nitric oxide can diffuse directly across cell membranes due to their small size and nonpolar nature. This is different from larger or polar molecules that require transport mechanisms.
3. Eliminate the incorrect options: Nitric oxide does not bind exclusively to intracellular steroid receptors, as it is not a steroid. It also does not act only through G protein-coupled receptors, nor is it stored in vesicles before release.
4. Focus on the correct characteristic of nitric oxide: it diffuses directly across cell membranes without requiring vesicles or transport proteins.
5. Conclude that the correct answer is: 'is a gas that diffuses directly across cell membranes,' as this property makes nitric oxide unusual among animal signal molecules.